The present invention relates to a lock-up control device of a torque converter used for an automatic transmission.
A torque converter drives a turbine by oil stirred by an impeller, and transmits power by hydraulic power transmission. Although the torque converter increases torque and absorbs torque fluctuations, slip occurs between the impeller and turbine, so efficiency of the transmission decreases.
In a lock-up torque converter, the slip can be eliminated and efficiency of the transmission can be increased by locking the impeller and turbine by engaging a lock-up clutch in a running region where torque increase and absorption of torque fluctuation is unnecessary.
The torque converter disclosed by JP-A-H5-79560 published by the Japanese Patent Office in 1993 discloses a torque converter comprising a lock-up control chamber partitioned by a lock-up clutch piston from a converter chamber. When a lock-up control pressure is supplied to this lock-up control chamber, the lock-up clutch piston displaces so that the lock-up clutch is engaged, and the impeller and turbine are locked. This torque converter is a three circuit lock-up torque converter comprising an inlet circuit which supplies oil to the torque-converter chamber, an outlet circuit which discharges oil from the torque-converter chamber, and a lock-up control circuit which supplies lock-up control pressure to the lock-up control chamber.
In the three circuit lock-up torque converter, the pressure (converter pressure) in the converter chamber is decreased by making the torque converter outlet circuit communicate with the drain port during lock-up, and the lock up control pressure can thereby be decreased.
However, if the torque converter outlet circuit is merely made to communicate with the drain port to decrease the converter pressure, oil balance (balance between the inflowing oil amount and outflowing oil amount) is impaired, and in particular when the pump is driven with the minimum necessary discharge amount to improve fuel cost performance, it may occur that oil is insufficient.
It is therefore an object of this invention to make effective use of the oil pressure discharged from a torque converter to decrease the converter pressure, and to prevent impairment of oil balance.
It is a further object of this invention to prevent seizure of an automatic transmission even under low temperature conditions when oil does not flow easily to an oil cooler for cooling during non lock-up periods.
It is yet a further object of this invention to prevent variation of pressure in the converter chamber during lock-up, and prevent a shock due to variation of the engaging force of a lock-up clutch.
In order to achieve above object, this invention provides a lock-up control device for controlling a lock-up clutch in a torque converter, the lock-up clutch being engaged by supplying a lock-up control pressure, and the torque converter transmitting torque via a fluid in a torque converter chamber when the lock-up clutch is disengaged and transmitting torque directly when the lock-up clutch is engaged. The lock-up control device decreases the fluid pressure in the torque converter chamber by discharging fluid in the torque converter chamber during lock-up, and leads the fluid discharged from the torque converter chamber to a lubricating part of the transmission.
According to an aspect of the invention, this invention provides a lock-up control device for controlling a lock-up clutch in a torque converter, the lock-up clutch being engaged by supplying a lock-up control pressure, the torque converter transmitting torque via a fluid in a torque converter chamber when the lock-up clutch is disengaged and transmitting torque directly when the lock-up clutch is engaged. The lock-up control device comprises a fluid circuit which decreases the flow pressure in the torque converter chamber by discharging fluid in the converter chamber during lock-up, and a lubricating circuit which leads the fluid discharged from the torque converter chamber to a lubricating part of the transmission.
The details as well as other features and advantages of this invention are set forth in the remainder of the specification and are shown in the accompanying drawings.